Push-to-talk over cellular (PoC), also referred to as “direct connect” is a service that permits a user or transmitter to transfer a voice message to one or more receivers simultaneously via a mobile telephone interface. Using PoC, the voice data is typically distributed over the mobile telephone communications network as the voice signal to be transmitted is spoken into the mobile telephone terminal by the user. From the user's point of view, using PoC is similar to using a CB radio or other similar device, but extended so that the transmitter can speak to receivers throughout the world, which can be reached via the switching technology of at least one mobile telephone communications network.
Generally, a user selects the people with whom he wishes to talk from a list on his mobile device. Once all of the selections are made, the selected people receive invitations on their mobile devices, which they may accept or decline. When the participants have accepted their invitations, the initial caller may press a “Push-and-Talk” (PaT) button on his mobile device to start the conversation, which is heard by all of the participants substantially simultaneously. Any participant in the call may reply immediately by pressing the PaT button, and speaking. Generally, communication can only go in one direction, so only one participant may speak at any given time. This is also know as half duplex call. Typically, whoever presses the PaT button first has the ability to talk for as long as the PaT button is held.
This mobile voice chat capability supplements SMS and eliminates some of the cumbersome steps required to set up a conference call. It is expected that this PoC service will replace the radio communications systems used by freight forwarders, courier services, taxi companies, building contractors, and others.
Push to talk supports three Kinds of calls: “Point to Point Call” in which a user can call single contact at a time; “Group Call” in which the group is a collection of contacts stored by a unique name in handset (a single group can be called using this feature, and the maximum number of contacts in a group is a configurable parameter); and “Adhoc Call” in which a user can call any number of groups and contacts together, and in which the maximum number of contacts allowed is again a configurable parameter.
In this context it should be noted that the term communication conference used in this document e.g. specifies a Group call or an Adhoc call which are half duplex in nature.
In many PoC systems, a standard protocol, known as Session Initiation Protocol (SIP) is used to set up a PoC communication link. SIP messages are sent from a client to a server, for the purpose of invoking particular operations. The server returns response codes to indicate the status of an operation to the client. In PoC systems Mobile phone always act as a SIP client as it always initiates request.
SIP requests and responses include header information that is used to identify and route requests and responses. This header information includes a “From” field that identifies the sender, a To field that identifies the intended recipient, a “Call-ID” field that identifies the call to which the message belongs, a “Cseq” field that contains a sequence number that is used for ordering messages, and other fields.
Each user in a SIP network is identified by a unique SIP address, which has a format similar to an e-mail address, such as “02114204701@gateway-r-us.com”. Addresses can also take the form of a “Tel:” URL for telephone numbers, such as “Tel:+0811797677”. A user's SIP address is generally registered with a registrar server, which assists in locating users when a SIP message needs to be directed to them.
The standard requests in SIP are shown in Table 1. These requests may include a variety of parameters.
TABLE 1SIP RequestsMessageFunctionINVITEUsed to invite a user or service to participate in asession.ACKConfirms that the client has received a final responseto an INVITE request.BYEIndicates to the server that the client wishes to releasethe call.CANCELCancels a pending request with specified Call-ID, To,From, and Cseq field values, but does not affect acompleted request.REGISTERRegisters an address listed in a To header field withthe SIP server,OPTIONSQueries the capabilities of the SIP server.
It should be noted that the requests listed in Table 1 are only the requests defined in RFC 3261, which is distributed by the Internet Engineering Task Force (IETF), and defines the basic functionality of SIP. A variety of extensions also exist, sonic of which are standard, and some of which are proprietary. These extensions may include new SIP requests, such as the SUBSCRIBE and NOTIFY messages, which are used for specific event notification, and are defined in IETF RFC 3265.
Standard SIP responses take the form of three-digit numbers, with the first digit determining the type of response, and the next two digits determining the specific response that is being sent. For example, all “1xx” responses are informational, “2xx” responses indicate success, “3xx” responses indicate that further action needs to be taken, “4xx” responses indicate a client error, “5xx” responses indicate a server error, and “6xx” responses indicate a global failure. Examples of specific responses are “181”, which indicates that a call is being forwarded, “200”, which indicates that an operation status is “OK”, and “503”, which indicates that a service is unavailable. A more complete list of recognized SIP response codes can be found, for example, in IETF RFC 3261.
The Push-to-Talk standard specifications are split between the Open Mobile Alliance (OMA) standardization gremium and the 3rd Generation Partnership Project/3rd Generation Partnership Project 2 (3GPP/3GPP2) standardization gremiums.
For mobile devices, such as phones with a Push-to-talk capability, the SIP messages and responses, as well as other data are transmitted over a General Packet Radio Service (GPRS) network or a Universal Mobile Telecommunications System (UMTS) network.
In case that in an already established communication conference one of the participants wants to create a new conference, according to PoC, the communication device will generate a list of other communication devices that should be invited to this new communication conference within the already established communication conference and transmit this list to the SIP conference server within a SIP INVITE message. The communication devices that should be invited to the new communication conference can be assigned individually by means of an individual communication device identifier or group-wise using predefined groups of a plurality of communication devices. The conference server would then generate a SIP INVITE message for each communication device contained in the received list.
What is needed in the art is a more efficient management of communication conferences.